1. Field of the Invention
The present invention relates to a liquid ejection head and an image forming apparatus comprising a liquid ejection head, and more particularly, to the stabilization of ink in a liquid ejection head capable of forming high-definition images.
2. Description of the Related Art
As an image forming apparatus in the related art, an inkjet printer (inkjet recording apparatus) is known, which includes an inkjet printer head (liquid ejection head) having an arrangement of a plurality of liquid ejection nozzles and which records images on a recording medium by ejecting ink (liquid) from the nozzles toward the recording medium while the relative movement between the inkjet head and the recording medium is performed.
The inkjet head of the inkjet printer ejects ink, for example, either by using piezoelectric elements or by generating bubbles by means of heating elements.
For example, in an inkjet head which ejects ink by generating bubbles by means of heating elements, the inkjet head generates bubbles by applying energy to heaters and causes ink droplets to be ejected by the pressure created by the bubbles, thereby recording images onto a recording medium. One of characteristics of such heads is their silent operation.
In an inkjet head of this kind, graduated tone recording is carried out in order to achieve high definition. More specifically, graduated tone recording is carried out by controlling the ejection volume of the ink. In this case, in a system based on heating elements in particular, it is difficult to achieve sufficient control of recording by means of nozzles having one and the same structure (a common structure). For example, when the ejection speed is adjusted on the basis of large liquid droplets in order for the large liquid droplets to be ejected at an appropriate speed, the actual ejection speed of small liquid droplets becomes slow and the ejection direction of small liquid droplets becomes unstable. Thus, it is difficult to obtain stable images. On the other hand, when the ejection speed is adjusted on the basis of small liquid droplets in order for the small liquid droplets to be ejected at an appropriate speed, the actual ejection speed of large liquid droplets becomes markedly high and rebounding of the droplets may occur when they deposit onto a recording medium. Thus, soiling of the image is caused.
Japanese Patent Application Publication No. 9-254413 discloses a method in which nozzles for ejecting large liquid droplets and nozzles for ejecting small liquid droplets are provided, and high-definition images are obtained by combining the liquid droplets ejected from these nozzles. More specifically, the nozzle diameters and heater sizes, and the like, of the nozzles for large liquid droplets differ from those of the nozzles for small liquid droplets. Based on such structure, the liquid droplets are ejected under optimal ejection conditions, in accordance with the size of the liquid droplets. Thus, a high-definition image can be obtained stably.
However, if a nozzle for ejecting large liquid droplets and a nozzle for ejecting small liquid droplets are provided as described in Japanese Patent Application Publication No. 9-254413, then new possibilities arise because of the difference between the usage frequencies of the nozzles.
More specifically, in cases where graduated tone recording for an image is carried out on the basis of area tones, small liquid droplets are principally used in order to raise tonal expressiveness. A large number of small liquid droplets are used especially in a region where there is marked change in the color tones of the image. On the other hand, large liquid droplets are principally used in a region where the color tones of the image are dark and where there is virtually no change in the color tones, namely, in a region having a large surface area of a single dark color, from viewpoints of reducing the number of ejection operations and the power consumption, and raising the printing speed.
In the cases where the nozzles for ejecting large liquid droplets and the nozzles for ejecting small liquid droplets are thus used selectively, the nozzles for ejecting large liquid droplets are used only for a region where the color tones of the image are dark and where there is little change in the color tones, namely, a region having a large surface area of a single dark color. Therefore, in the case of general high-definition images, the usage frequency of the nozzles for ejecting small liquid droplets tends to be higher than that of the nozzles for ejecting large liquid droplets. If ink inside the nozzles is not used for a long period of time, then the solvent contained in the ink evaporates and the viscosity of the ink increases. Since usage frequency of nozzles for ejecting large liquid droplets is thus different from usage frequency of the nozzles for ejecting small liquid droplets, then the viscosity of the ink in the vicinity of the nozzles having a low usage frequency, namely, the nozzles for ejecting large liquid droplets, becomes greater than the viscosity of the ink in the vicinity of the nozzles for ejecting small liquid droplets.
This situation is described more specifically with reference to FIG. 11. FIG. 11 is a cross-sectional diagram showing a liquid ejection head which generates bubbles by means of heat generated from heating elements (heaters) and thereby ejects ink from nozzles for ejecting small liquid droplets and nozzles for ejecting large liquid droplets.
A nozzle (small nozzle) 91 for ejecting small liquid droplets and a nozzle (large nozzle) 92 for ejecting large liquid droplets are provided in a nozzle plate 90, and heaters 95 and 96 corresponding to the nozzles 91 and 92 (small nozzle and large nozzle) are provided across an ink flow channel 93 from the nozzles 91 and 92, respectively. Usage frequency of the small nozzle 91 is high during the operation of the image formation apparatus, and ink flows constantly into the small nozzle 91 from the ink flow channel 93. Therefore, the viscosity of the ink inside the nozzle 91 for ejecting small liquid droplets does not increase. On the other hand, usage frequency of the large nozzle 92 is not high, and therefore ink which has flowed in from the ink flow channel 93 and which fills the large nozzle 92 remains in the nozzle 92 for a long period of time. Therefore, the viscosity of ink in the vicinity of the large nozzle 92 increases gradually and the increased viscosity region 94 of the ink extends progressively from the large nozzle 92 and the vicinity thereof, to the ink flow channel 93, as indicated by the arrows in FIG. 11.
In cases where viscosity of the ink inside the nozzles has increased, it is necessary to carry out a suctioning operation as described below. If the increased viscosity region 94 of the ink has extended into the ink flow channel 93 from the large nozzle 92, then it is necessary to suction a large amount of ink in order to remove the ink in the increased viscosity region 94, and hence a large amount of ink is wasted.
Furthermore, the suctioning operation is carried out by means of a common suctioning cap, at the same suctioning pressure for both the large nozzle 92 and the small nozzle 91. There is a difference in the aperture diameter between the large nozzle 92 and the small nozzle 91, and the flow resistance in the large nozzle 92 is lower than that in the small nozzle 91. Therefore, the amount of ink suctioned from the large nozzle 92 is large and wasteful consumption of ink occurs.